A hydraulic reaction mechanism of this kind is disclosed in, for example, U.S. Pat. No. 4,434,866 issued to James J. Duffy. As shown in FIG. 1, this mechanism includes an input shaft 2 having radially protruding portions 2a. A sleeve 3 rotates with an output shaft (not shown), and is provided with grooves 3a in which the protruding portions 2a are received so as to be rotatable through a given angle. The power steering system with which this reaction mechanism is used has a housing 1. The sleeve 3 is fitted in the housing 1 in such a way that the outer periphery 3b of the sleeve can angularly move along the inner surface 1a of the housing 1. Reaction pistons 4 are fitted in the sleeve 3 to hydraulically push the sides of the protruding portions 2a in directions perpendicular to the sides. Since this reaction mechanism does not produce a large frictional loss, it operates efficiently by itself, and develops only a little hysteresis
However, this reaction mechanism has the following disadvantages. First, since the reaction pistons 4 are simply inserted in their respective cylinder chambers 5 formed within the sleeve 3 so as to be slidable, when the sleeve 3 is inserted into the housing 1, the pistons 4 readily come off. Thus, the mechanism cannot be assembled in an efficient manner. Secondly, a step is formed on the recessed wall of each cylinder chamber 5 to limit the forward movement of each reaction piston. Therefore, the cylinder chambers 5 are cumbersome to machine.